One of the known methods for obtaining structural information about the molecular ions by mass spectrometry is an MS/MS analysis (or MSn analysis). In a typical MS/MS analysis, an ion having a desired mass-to-charge ratio is first separated from the material to be analyzed. This ion is called the precursor ion, or the parent ion. Next, the precursor ion thus separated is broken into fragment ions by a collision-induced dissociation (CID) process. Finally, the fragment ions (or daughter ions) produced by the dissociation process, are mass-analyzed to obtain a mass spectrum, which provides information about the molecular structure of the precursor ion.
In the CID process, the sample ion collides with a gas (called the target gas) within a collision section, whereby the collision energy breaks the sample ion into smaller ions. Some of the devices for the MS/MS (or MSn) analysis include multiple mass separators connected in series, and some others use an ion trap for capturing and breaking a specific ion. In the former type, a collision cell is located between the two neighboring mass separators as the collision section, whereas, in the latter type, the ion trap having an inner ion-trapping space serves as the collision section (see Patent Documents 1 and 2). If a time-of-flight mass spectrometer is used, the collision section may be located at a specific section of the flight tube (see Patent Document 3).
If a collision cell is provided as the collision section, the target gas is introduced into the collision cell and a precursor ion is supplied into the same cell. Then, the precursor ion passing through the collision cell collides with the target gas, and the precursor ion is broken into fragment ions. If an ion trap is used as the collision section, the target gas is introduced into the ion trap, in which the target gas collides with the precursor ions having a specific range of mass-to-charge ratios being gathered at the center by an electric field created within the ion trap. Thus, the ions are broken into fragment ions. If a section of the flight tube is used as the collision section, the target gas collides with the precursor ion when the ion passes through the collision section, thereby breaking the precursor ion into fragment ions.
[Patent Document 1] U.S. Pat. No. 4,234,791
[Patent Document 2] Unexamined Japanese Patent Publication No. 2002-184349
[Patent Document 3] U.S. Pat. No. 5,202,563
[Non-Patent Document 1] Yasuo SHIDA, et al., Korenara Wakaru Masu-Supekutorometorii (For Learners of Mass Spectrometry), Kagaku-dojin Publishing Company, Kyoto, 2001, pp. 46-51
In general, a mass spectrometer using a conventional CID process to break the precursor ion uses argon, helium or some other inert gas atom as the target gas to be introduced into the collision section (see Non-Patent Document 1). However, since the mass of the inert gas atom is small, the CID process using the inert gas cannot break a large molecule having a molecular weight of about 5000 Da or larger. This means that the conventional method cannot provide information about the structure of such a large molecule.
Accordingly, the main objective of the present invention is to provide a mass spectrometer capable of dissociating even precursor ions having a large molecular weight by the CID process.